1. Field of the Invention
This invention relates to an accelerometer capable of measuring acceleration along two axes.
2. Description of the Related Art
Accelerometers are well known in the art, and are used in many applications, such as aeronautic and automotive applications, in order to detect rapid acceleration or deceleration. One example of an application of an accelerometer is that of air bag deployment in automobiles.
As control systems have become more sophisticated it has become desirable to detect acceleration/deceleration in more than one direction so that control of, for example, an air bag, can be adapted dependent upon the direction of acceleration/deceleration. This is difficult to achieve, however. One approach is simply to provide two accelerometers one for each of the two axes of interest. Accurate alignment is difficult, however, and the resulting device is bulky and expensive. An alternative is to attempt to fabricate a device which has acceleration/deceleration sensitivity in two directions. Such devices, are, however, difficult to manufacture, and tend to have cross-axis sensitivity, such that acceleration along one axis affects the output indicative of acceleration in the other axis, resulting in erroneous readings. Furthermore, with such devices it is often difficult to alter the sensitivity along one axis without altering that of the other axis.
The present invention seeks to provide an accelerometer which is capable of detection of acceleration along two axes, and which is compact yet simple to manufacture. Furthermore, the present invention seeks to provide an accelerometer which has minimal cross-axis sensitivity.
According to the present invention there is provided an accelerometer comprising:
a housing including a mass-supporting frame; and
a mass supported on the frame;
support means for supporting the mass on the frame;
means for detecting rotation of the mass with respect to at least part of the support means; and
means for detecting translation of the mass with respect to at least part of the support means.
The mass may comprise an outer mass connected to a pair of outer spring members connected to the frame and an inner mass connected to the outer mass by one or more inner spring members. The spring members in this case form the support means. The axes of the one of more inner spring members may define a plane with which the axis of the outer spring members also may be aligned. In this case, the means for detecting torsional stress may consist of means for detecting torsion stress induced in the outer spring members during acceleration of the accelerometer in a direction which has a component perpendicular to the first axis in the plane of the inner and outer spring members; and
the means for detecting translation may consist of means for detecting bending stress in the inner spring members during acceleration of the accelerometer in a direction perpendicular to the plane of the spring members.
Preferably in this case there will be two pairs of inner spring members, and the axis through one pair of the inner spring members will be perpendicular to the axis through the other pair. The inner mass is supported by these two pairs of inner spring members. The axis of the inner spring members and the axis of the outer spring members may be at substantially 45xc2x0 to one another. Said axes define a plane.
Preferably, the accelerometer is formed from semiconductor material and may be a material with anisotropic properties; preferably silicon.
By employing torsional stress for one direction of detection and bending stress for the other it is possible to minimise cross-axis sensitivity. Preferably the means for determining torsional stress in the support means is positioned in an area on one or more of the outer spring members which is substantially half way between the supporting frame and the mass. Preferably, the means for determining the bending stress in the support means is positioned either on one or both of a pairs of inner spring members adjacent to an inner or outer parting of the mass.
Preferably, the device is formed from silicon and the means for determining the torsional stress may comprise a Wheatstone bridge formed from four piezoresistors, each of the piezoresistors arranged on the support means substantially along a (110) crystal axis in silicon.
The device may be formed from silicon and the means for determining bending stress may comprise two piezoresistors on each of the inner spring members, one at each end of each spring member, or alternatively, may comprise a piezoresistor at one end of each inner spring member, if at least two pairs of inner spring members are provided. All piezoresistors on the inner spring members are preferably but not necessarily aligned parallel to each other and along a  less than 110 greater than  crystal axis silicon.
By providing an inner mass and an outer mass it is possible to alter the sensitivity to acceleration in the in-plane direction simply by altering the outer mass, leaving the sensitivity in the other axis unaltered. Furthermore, by changing the ratio of outer mass to inner mass, without, altering total mass, the sensitivity in one direction is unchanged whilst that in the other direction is changed. This leads to considerable design flexibility.
By arranging for the outer spring members and inner spring members to be offset by an angle of substantially 45xc2x0 it is possible for the device to take advantage of the variation in the piezoresistive coefficient of a semiconductor material of cubic symmetry such as silicon with respect to alignment of the crystal planes of the semiconductor from which the accelerometer is formed. This minimizes the cross-axis sensitivity of the device.